A prior art electronic ballast for a discharge lamp has been designed to assemble various electric parts forming the circuits of the ballast into a single casing. In the prior ballast, it has been a practice to use a circuit board which mounts on its one surface the electric parts and is assembled into the casing. When the ballast is required to have an added function of, for example, controlling the operation of the discharge lamp based upon the operating condition of the discharge lamp, the circuit board is designed to mount a corresponding number of parts forming a detection circuit of detecting the operating condition of the discharge lamp, and therefore requires an extra space for accommodating the parts of the detection circuit. Further, when the ballast is designed to have an inverter with an output transformer which is inherently large, the circuit board has to be correspondingly large enough to mount the output transformer together with the other electric parts. Accordingly, the prior art ballast has an underlying problem in that it is difficult to be made compact for easy installation in a limited space when the ballast is equipped with the added function of controlling the operation of the discharge lamp based upon the detected operating condition thereof and includes the output transformer.